For the past year, rebel groups that took their war to Syria's main cities have been unable to finish what they started. Pinned down by air power and shelling that can hit them wherever they are, opposition fighters have barely advanced in Aleppo and Damascus; their gains in some areas have been offset by losses in others.

With grinding stalemate now the undeniable reality, the rebels have pleaded for weapons that could give them a tactical edge. While they have enough Kalashnikovs and ammunition to keep the battle going, they don't have the firepower they need to win it.

Top of their wish list has been anti-aircraft weapons, particularly missiles such as the shoulder launched heat seekers that they have managed to steal from regime military bases and used to down an estimated 15 Mig fighters and attack helicopters. No more than a handful of such weapons have been smuggled into Syria – many more have been confiscated by Jordanian and Turkish intelligence officers, much to the chagrin of myriad gunrunners.

Most, if not all of the seized heat seekers had been sourced from Libyan arms depots looted after the fall of Muammar Gaddafi. Under heavy US pressure, Qatar and Saudi Arabia, who recently stepped up their direct support for some groups, have been reluctant to give them what they want.

The Americans' fears, seconded by Israel, are based on a repeat of the Afghanistan scenario of the late 80s, in which surface-to-air missiles supplied by the CIA to combat the Soviet air force fell into the hands of groups who might have other targets in mind, such as civilian aircraft.

The patchwork of Syria's opposition fighting groups cannot speak with a common voice. The jihadist groups among them – at first an insignificant rabble and now a sizable, potent minority – have very different aims to those groups fighting a nationalistic cause. Keeping such weapons out of their hands is more important to the US in particular than deciding who might be worthy of support.

To that end, a reluctant US administration has lately settled on Salam Idriss, the commander of the umbrella guerilla group the Free Syria Army, as a leader for whom it is prepared to take a risk. Dealings between Idriss and the US military have stepped up in recent months, both in Jordan and Turkey, where small groups of Syrian rebels are being trained.

An influx of Saudi-supplied weapons that crossed the Jordanian border earlier this year were channelled through vetted Idriss loyalists. The supply included explosives that can take out tanks and cause extensive damage to structures as rebels advance. But it did not include the holy grail of heat seekers.

When John McCain, an outspoken advocate of direct US intervention in Syria's civil war, crossed the Turkish border into the rebel heartland of northern Syria earlier this week, he repeated his insistence that more had to be done to take the fight to the Assad regime and that the international community had an obligation to act. McCain seems as sure as he can be that an arming effort could be managed to stop powerful weapons falling into the hands of al-Qaida-aligned groups that may have a different purpose in mind for them.

The veteran senator and former presidential candidate is clearly conscious of the dangers of overreaching. While he endorses the supply of stalemate-breaking weapons to rebel depots, he has so far drawn the line on missiles that can down planes. McCain well knows the damage caused by the last US lawmaker to wander around an insurgency offering heat seekers – Charlie Wilson in the early 1980s. Syria now is as volatile as Afghanistan was back then. "John McCain's War" remains a difficult sell.

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